1. Field of the Invention
The present disclosure relates to a control apparatus in a motor drive system and a method of controlling the motor drive system.
2. Discussion of the Background
Motors serving as drive sources are provided in vehicles, such as hybrid electric vehicles (HEVs) and electric vehicles (EVs). FIG. 8 is a partial cross-sectional view of a permanent magnet field motor. A motor 50 illustrated in FIG. 8 includes a rotor 53 including multiple permanent magnets 51 provided around a rotating shaft in a concentric manner and a stator 57 including three-phase armatures 55 provided on an outer peripheral side of the rotor 53.
For example, refer to Japanese Patent No. 3746334.
When the motor 50 illustrated in FIG. 8 is operated at high temperature of the permanent magnets, demagnetization occurs in the permanent magnets 51 due to a demagnetizing field of phase current involved in the operation at high temperature. The occurrence of the demagnetization in the permanent magnets 51 may reduce the torque in the motor 50 to prevent the motor 50 from outputting a desired output. As a countermeasure against the problem, a method is proposed in which armature current is controlled so that the temperature (hereinafter referred to as a “magnet temperature”) of the permanent magnets does not exceed an allowable maximum temperature. For example, a rotating electrical machine apparatus for a hybrid vehicle described in Japanese Unexamined Patent Application Publication No. 2001-157304 estimates the magnet temperature of the rotating electrical machine on the basis of data detected to be used in control of the hybrid vehicle or data calculated in the apparatus on the basis of the detected data and, if the magnet temperature is high, limits the armature current. Although the limitation of the armature current in the above manner results in a reduction in output from the motor although the limitation of the armature current in the above manner allows an increase in the magnet temperature to be inhibited to suppress the occurrence of the demagnetization in the permanent magnets.
As a countermeasure against a reduction in output from the motor due to the demagnetization of the permanent magnets, a method of performing so-called field weakening control is also proposed to control the armature current so as not to cause the demagnetization. However, the output efficiency is reduced in the field weakening control because field weakening current, which is a current component that does not contribute to or is difficult to contribute to an increase in torque, is supplied to the motor. Alternatively, a method is also proposed in which a permanent magnet containing a larger amount of a heavy rare earth element, such dysprosium (Dy) or terbium (Tb), is used to suppress the demagnetization. However, since the heavy rare earth element is rare, it is not desired to increase the amount of use of the heavy rare earth element.